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Thierry Dupont, Matthieu Plu, Philippe Caroff, and Ghislain Faure

Abstract

Several tropical cyclone forecasting centers issue uncertainty information with regard to their official track forecasts, generally using the climatological distribution of position error. However, such methods are not able to convey information that depends on the situation. The purpose of the present study is to assess the skill of the Ensemble Prediction System (EPS) from the European Centre for Medium-Range Weather Forecasts (ECMWF) at measuring the uncertainty of up to 3-day track forecasts issued by the Regional Specialized Meteorological Centre (RSMC) La Réunion in the southwestern Indian Ocean. The dispersion of cyclone positions in the EPS is extracted and translated at the RSMC forecast position. The verification relies on existing methods for probabilistic forecasts that are presently adapted to a cyclone-position metric. First, the probability distribution of forecast positions is compared to the climatological distribution using Brier scores. The probabilistic forecasts have better scores than the climatology, particularly after applying a simple calibration scheme. Second, uncertainty circles are built by fixing the probability at 75%. Their skill at detecting small and large error values is assessed. The circles have some skill for large errors up to the 3-day forecast (and maybe after); but the detection of small radii is skillful only up to 2-day forecasts. The applied methodology may be used to assess and to compare the skill of different probabilistic forecasting systems of cyclone position.

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Marie-Dominique Leroux, Julien Meister, Dominique Mekies, Annie-Laure Dorla, and Philippe Caroff

Abstract

A 17-yr “climatology” of tropical-system activity, track, size, and 24-h intensity change in the southwest Indian Ocean (SWIO) is developed and analyzed in comparison with other intensively studied basins such as the North Atlantic Ocean. A first formulation of the empirical maximum potential intensity of SWIO tropical systems is also proposed, along with the climatology of sea surface temperatures from September to June. Systems with a 34-kt (1 kt = 0.514 m s−1) wind radius that does not exceed 46 km are considered to be very small or midget systems, on the basis of the 5th percentile of storm size distribution. Using the 95th percentile of overwater intensity changes, rapid intensification (RI) is statistically defined by a minimum increase of 15.4 m s−1 day−1 in the maximum 10-min mean surface wind speed (VMAX). This value is similar to the 30-kt threshold commonly used in the North Atlantic basin for 1-min sustained wind speeds. Rapid decay (RD) can be statistically defined by a minimum weakening of 13.9 m s−1 day−1, although the spread in the 5th percentile of intensity changes among the different intensity classes indicates that it is not as appropriate to use a unique RD threshold for all systems. It is shown that 43% of all tropical systems and all very intense tropical cyclones (VMAX ≥ 59.6 m s−1) underwent RI at least once during their lifetimes. It is highlighted that systems have a greater propensity to intensify rapidly for an initial intensity between 65 and 75 kt. Statistics indicate that operational intensity forecast errors are significantly greater at short range for RI cases while track errors are reduced.

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Kerry Emanuel, Philippe Caroff, Sandy Delgado, Charles “Chip” Guard, Mark Guishard, Christopher Hennon, John Knaff, Kenneth R. Knapp, James Kossin, Carl Schreck, Christopher Velden, and Jonathan Vigh
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Christopher Velden, Bruce Harper, Frank Wells, John L. Beven II, Ray Zehr, Timothy Olander, Max Mayfield, Charles“Chip” Guard, Mark Lander, Roger Edson, Lixion Avila, Andrew Burton, Mike Turk, Akihiro Kikuchi, Adam Christian, Philippe Caroff, and Paul McCrone
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Christopher Velden, Bruce Harper, Frank Wells, John L. Beven II, Ray Zehr, Timothy Olander, Max Mayfield, Charles “CHIP” Guard, Mark Lander, Roger Edson, Lixion Avila, Andrew Burton, Mike Turk, Akihiro Kikuchi, Adam Christian, Philippe Caroff, and Paul McCrone

The history of meteorology has taught us that weather analysis and prediction usually advances by a series of small, progressive studies. Occasionally, however, a special body of work can accelerate this process. When that work pertains to high-impact weather events that can affect large populations, it is especially notable. In this paper we review the contributions by Vernon F. Dvorak, whose innovations using satellite observations of cloud patterns fundamentally enhanced the ability to monitor tropical cyclones on a global scale. We discuss how his original technique has progressed, and the ways in which new spaceborne instruments are being employed to complement Dvorak's original visions.

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